The present invention relates to a process for improving the polymerization of olefins and more particularly to the utilization of an oxygen-containing gas in an olefin polymerization process which employs a supported organophosphoryl-chromium oxide catalyst system.
Numerous processes are known for the polymerization of olefins utilizing chromium oxide supported catalysts. For example, U.S. Pat. Nos. 3,985,676 and 4,192,775 disclose excellent supported catalysts systems prepared by depositing preformed materials comprising the reaction product of organophosphorus compounds, such as organophosphates and organophosphites, with chromium trioxide. Such catalysts provide excellent resin products, particularly polyethylenes having very desirable physical properties.
Notwithstanding, the excellence of these catalysts systems, some difficutly has been observed occasionally in the producing resin of a desired density.
The literature, including the patent literature, is replete with methods for improving olefin polymerization process. For example, numerous U.S. patents including U.S. Pat. Nos. 2,994,691, 2,868,772 and British Pat. No. 471,590 add oxygen to the polymerization medium for various reasons. In particular, U.S. Pat. No. 2,994,691 discloses the use of oxygen or an oxygen-containing gas such as air in contact with the reaction medium of a catalyst system comprised of a transition metal halide such as titanium tetrachloride and an organometallic, such as a trialkylaluminum, and an alpha-olefin to increase the speed of polymerization and the yield of polymer produced per unit of catalyst. Similarly, U.S. Pat. No. 2,868,772 discloses the addition of an oxygen containing component such as molecular oxygen or hydrogen peroxide, to catalyst systems comprised of transition metal halides and organometallics useful in the production of alpha-olefin polymers having relatively uniform molecular weights. British Pat. No. 471,590 illustrates an early recognition of the role of oxygen in high pressure olefin polymerization processes.
Moreover, the use of oxygen or air in the activation of supported chromium oxide catalysts is also well known as evidenced for example by U.S. Pat. Nos. 3,985,676, 4,192,775, 3,349,067, 3,541,072, 3,870,693, 3,346,511, 2,945,015 and 3,365,439.